The Collaborative International Dictionary of English v.0.48:

Turnsole \Turn"sole`\, n. [F. tournesol, It. tornasole; tornare
   to turn (LL. tornare) + sole the sun, L. sol. See Turn,
   Solar, a., and cf. Heliotrope.] [Written also turnsol.]
   1. (Bot.)
      (a) A plant of the genus Heliotropium; heliotrope; -- so
          named because its flowers are supposed to turn toward
          the sun.
      (b) The sunflower.
      (c) A kind of spurge (Euphorbia Helioscopia).
      (d) The euphorbiaceous plant Chrozophora tinctoria.
          [1913 Webster]

   2. (Chem.)
      (a) Litmus. [Obs.]
      (b) A purple dye obtained from the plant turnsole. See
          def. 1
      (d) .
          [1913 Webster]